Keith is hairy Mike is insane Ossie is hairy Noone is green 

is proved invalid by  
Jeff is green Ossie is green Everything is green 

is proved invalid by 
Jeff is not a kangaroo A kangaroo exists 

is proved invalid by 

Nigel is an elk A nonelk exists 

is proved invalid by 

Is x(Ax) true here? Is ~x(~Ax) true here? 
Is x(Ax) false here? Is ~x(~Ax) false here? 

Is x(Bx) true here? Is ~x(~Bx) true here? 
Is x(Bx) false here? Is ~x(~Bx) false here? 

Is ~x(Bx) true here? Is x(~Bx) true here? 
Is ~x(Bx) false here? Is x~(Bx) false here? 

Is ~x(Cx) true here? Is x(~Cx) true here? 
Is ~x(Cx) false here? Is x(~Cx) false here? 
Practice 12. Use your own paper or the answer sheet at Practice Sheet. For each of the following arguments do TWO things. First, translate it into English based on the following scheme.
Second, circle it if it's valid, cross it out if it's invalid.
